A common technique for indirect blood pressure measurement depends on an occlusive cuff and a stethoscope for ausculation of Korotkov sounds. While satisfactory for many clinical purposes, the technique is subject to errors due to such factors as stethoscope efficiency, the hearing acuity of the examiner, ambient noise, and the variability in Korotkov sound intensity and spectrum between individuals. In addition, a stethoscope may be inconvenient for extended or ambulatory blood pressure monitoring.
The stethoscope may be replaced with an electrical transducer such as a microphone, piezoelectric sensor or strain gauge. A typical piezoelectric sensor consists of a small piece of piezo-ceramic material, such as lead-zirconate-titanate (PZT), cemented to the center of a larger circular metal plate. The sensitivity of this transducer is dependent on the excitation point of the metal plate. Maximum output is realized if the plate is excited at the center, but the output decreases sharply as the excitation point is shifted outward toward the edge of the plate.
Due to non-uniform sensitivity, the piezoelectric sensor must be positioned accurately over the brachial artery. If the sensor is shifted even a small amount (a not unlikely occurrence under ordinary conditions of use), the output may drop or cease entirely. The problem is aggravated for patients with obese or flabby arms, ambulatory monitoring, and patients with a weak brachial pulse. Strain gauge and microphone transducers may have similar limitations.
It is thus an object of the invention to provide a blood pressure transducer which is more accurate under a variety of clinical conditions.